Pinatubo eruption in 1991

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The second-largest volcanic eruption of the 20th century, and by far the largest eruption to affect a densely populated area, occurred at Mount Pinatubo in the Philippines on June 15, 1991. The eruption produced high-speed avalanches of hot ash and gas, giant mudflows, and a cloud of volcanic ash hundreds of miles across.

In early June, tiltmeter measurements had shown that the volcano was inflating, evidently due to growing amounts of magma filling the reservoir beneath the summit. At the same time, seismic activity, previously concentrated at a depth of a few kilometers below a point about 5 kilometres (3.1 mi) northwest of the summit, shifted to shallow depths just below the summit.

On June 7, the first magmatic eruptions took place with the formation of a lava dome at the summit of the volcano. The dome grew substantially over the next five days, reaching a maximum diameter of about 200 m (660 ft) and a height of 40 m (130 ft).

Timely forecasts of an eruption by scientists from the Philippine Institute of Volcanology and Seismology and the U.S. Geological Survey enabled people living near the volcano to evacuate to safer distances, saving at least 5,000 lives.

A few hours later the same day, large explosions lasting about half an hour generated an eruption column which quickly reached heights of over 19 kilometres (12 mi), and which generated pyroclastic flows extending up to 4 km (2.5 mi) from the summit in some river valleys. Fourteen hours later, a 15-minute eruption hurled ash to heights of 24 km (15 mi). Friction in the uprushing ash column generated abundant lightning.

A third large eruption began at 08:41 on June 13, after an intense swarm of small earthquakes over the previous two hours. It lasted about five minutes, and the eruption column once again reached 24 km. After three hours of quiet, seismic activity began, growing more and more intense over the next 24 hours, until a three-minute eruption generated a 21 km (13 mi) high eruption column at 13:09 on June 14.

Tephra fall from these four large eruptions was extensive to the southwest of the volcano. Two hours after the last of these four explosions, a series of eruptions began which lasted for the next 24 hours, and which saw the production of much larger pyroclastic flows and surges which traveled several kilometres down river valleys on the flanks of the volcano.

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